322 IMPREGNATING OILS The specific gravity of soft pitches is usually 1-250-1-265, that of hard 1-275-1-280, and that of very hard 1-275-1-280. For pitch from gas tar, the carbon content is rarely less than 25-30%, and for that from vertical retorts or blast-furnaces, 5-7%. The ash content is less than 0-5% for gas pitch and more than i% (6-10%) for that from blast-furnaces. The yield of coke varies from 30 to 60%, and the coke has a more or less porous appearance according to the type of pitch from which it is derived (very porous with the very hard pitches, less so with the others). As regards the temperatures of softening and fusion, the following limits may be taken for different types of pitch : Soft: Softens at 40°, melts at 50-60°. Hard : Softens at 60°, melts at 70-80°. Very hard : Softens at 80-85°, melts at 90-120°. A good pitch for making briquettes should, according to Spilker 1 have the following properties : not more than 0-5% of ash ; softening point between 60° and 75° ; solubility in aniline, 70-75%, and in carbon disulphide, not less, than 70% ; yield of coke, 45% ; appearance of coke, caking and not too much swollen. IMPREGNATING OILS These are usually creosote oils or anthracene oils, freed more or less completely from crystallisable substances, and are used for the impregna- tion of wood, especially railway sleepers and telegraph poles, with the object of preserving it. In general they are brownish red or blackish liquids, more or less fluores- cent, somewhat viscous, and with a more or less marked odour of the products of tar distillation. The principal tests to be made are : 1. Determination of the Specific Gravity.—By means of a hydro- meter or Westphal balance at 15° C. In some cases measurements are made at higher temperatures, e.g., at 25, 45, 50° C. ; the temperature used must be indicated in the report. 2. Distillation.—This is carried out in a tubulated retort of about 300 c.c. capacity, furnished with a thermometer. The retort is charged with loo c.c. of the liquid and the thermometer bulb arranged at about 2 cm. from the liquid, which is heated so that 120 drops per minute pass over; the different fractions are measured. 3. Determination of the Phenols and the Naphthalene.—The fractions obtained as under 2 are reunited in a graduated cylinder, shaken repeatedly with 100 c.c. of caustic soda solution^(D 1*15) saturated with sodium chloride and then left to settle ; the increase in volume of the soda solution gives the percentage of phenols. In the supernatant oily layer the naphthalene is determined by cooling (to 15°) in the manner indicated for middle oils. 4. Test for the Presence of Solid Substances.—20 c.c. should remain liquid when heated to 40° and when shaken with 20 c.c. of pure benzene: when filtered through paper, the solution thus obtained should not leave a brown mark on the filter. 1 Lunge: Coal-Tar and Ammonia (London, 1916), p. 542.